The present disclosure relates to a light guide plate preferable for application to a backlight unit in, particularly, a transmission type liquid crystal display, to a backlight unit using the light guide plate and a method of manufacturing the same, and to a liquid crystal display using the backlight unit.
In recent years, very thin type displays such as CRTs (Cathode Ray Tubes), LCDs (Liquid Crystal Displays) and PDPs (Plasma Display Panels) have been proposed as displays for television sets and been put to practical use. Particularly, the liquid crystal displays using liquid crystal panels have spread more and more and technical research and development thereof have been progressing because the liquid crystal displays can be driven with low power consumption and the prices of large-type liquid crystal panels have been decreasing.
In the liquid crystal displays, the backlight system has become a main stream, where a transmission type liquid crystal display panel equipped with color filters is illuminated from the back side by a backlight unit capable of plane-mode illumination, whereby a color image is displayed.
As a light source for the backlight unit, a CCFL (Cold Cathode Fluorescent Lamp), which emits white light by use of a fluorescent lamp, and LEDs (Light Emitting Diodes) are deemed as promising (refer to, for example, Japanese Patent Laid-open No. Hei 8-136917).
Particularly, owing to the development of the blue light emitting diode, the set of LEDs for respectively emitting red light, green light and blue light, which are the three primary colors of light, has been completed, and it is possible to obtain white light with high color purity by mixing the red light, green light and blue light emitted from the LEDs. Therefore, the use of a set of the LEDs as a light source for a backlight unit makes it possible to enhance the color purity obtained through the liquid crystal display panel and, therefore, to largely broaden the range of reproducible colors, as compared with that attained by use of a CCFL. Furthermore, the use of high-output LED chips makes it possible to greatly enhance the luminance of the backlight unit.
In the case of using a set of the LEDs as a light source of a backlight unit, if the layout of the LEDs is of the beneath type, i.e., if the LEDs are laid out beneath the light emitting surface, luminance irregularities and/or color irregularities would be generated because the LEDs are point light sources.
In order to solve this problem, the distance between the LEDs and a diffuser plate may be enlarged. This approach, however, results in the backlight having a very large thickness, as compared with a backlight using the CCFL. The liquid crystal displays are desired to be further reduced in weight and thickness, and the backlight therefor is also desired to be further reduced in thickness.
In view of this, as a method for reducing the thickness of backlight, there has been proposed a structure in which a light guide plate is used and the light from a light source disposed at a part of the light guide plate is subjected to multiple reflection so as to form a surface light source.
In addition, as an LED for use as a light source, there has been proposed a structure of using a side emitter type LED in which the lens covering a light emitting device is so shaped that the light emitted just above is refracted into a lateral direction and the light is taken out principally in the lateral direction (refer to, for example, Nikkei Electronics (Nikkei Business Publications, Inc.), Dec. 20, 2004 (No. 889), pp. 123 to 130).
A backlight unit using such side emitter type LEDs is useful because it ensures that mixing of RGB (red, green and blue) lights is easy to carry out and that luminance irregularities and color irregularities can be restrained.
However, even if the LEDs radiating light primarily in a lateral direction as mentioned above are used, the light emitted from the LEDs in the vertically upward direction may not be completely eliminated.
Therefore, for example, a general sectional configuration is schematically shown in FIG. 18. A light guide plate 30 is provided with a light source hole part 31 communicating from a light emitting surface 30A thereof to a back surface 30B thereof, and a light source 21 composed, for example, a side emitter type LED or the like is disposed on a reflecting structure part 22 on the back surface 30B side in the light source hole part 31. A light transmitting member 71 formed with a light-shielding part 72 of a white ink or the like by printing or the like is formed, and the light transmitting member 71 is so disposed that the light-shielding part 72 is opposed to an upper portion, or the light outgoing side, of the light source hole part 21 of the light guide plate 30.
In this case, however, it may be necessary for the light-shielding part 72 to be formed at a predetermined position, or the position corresponding to the layout pattern of the light source hole part 21, of the light transmitting member 71. This configuration is disadvantageous for reductions in cost and in the number of component parts.
Besides, in the case of using red, blue and green LEDs as light sources, if the light going out of the LED in the vertically upward direction is shielded by use of such a light-shielding member, the other-colored light coming from the surroundings may mix into the light under consideration, possibly leading to the generation of color irregularities.
Therefore, the simple provision of a light-shielding structure directly above the light source is unsatisfactory for a display. Specifically, it is difficult to completely eliminate the luminescent point, or the so-called hot spot, due to the leakage of light from a roughly central position of the LED, and to eliminate luminance irregularities and color irregularities. At present, it may be necessary to provide a comparatively large spacing between the light sources and the display panel for sufficiently reducing the luminance irregularities and color irregularities, and it is difficult to make a backlight unit and a liquid crystal display smaller in thickness. In addition, depending on the materials of a reflective coat or a light absorber used as the light-shielding part, the amount of light absorbed and the amount of unavailable light may be large, which may result in lowering in light take-out efficiency and in light utilizing efficiency.
In order to solve these problems, it may be said to be desirable to direct more of the light going out of the light source vertically upwards into the light guide plate, rather than to simply shield the upper side of the light source.
Besides, not only in the case of using the side emitter type LEDs as above-mentioned but also in the case of using ordinary LEDs, if the light going upwards from the LED can be guided into the light guide plate, it is possible to reduce the number of component parts, to improve the luminance irregularities and color irregularities, and further to reduce cost.
Thus, there is a need for a light guide plate capable of guiding more of the light emitted from a light source vertically upwards, or toward the light emitting surface side, into the light guide plate and capable of reducing the number of component parts, for a backlight unit using the light guide plate and a method of manufacturing the backlight unit, and for a liquid crystal display using the backlight unit.